I think sometimes the long & involved expensive diagnosis is required to get school accommodations (but hopefully the student doesn’t pay for it).
For regular folks looking for a diagnosis and meds, the cost should just be the normal cost of an appointment with a psychiatrist or an MD. In an hour, they ask you a bunch of questions and decided if they think that is the best direction to go – it can be less than satisfying, especially if the person doesn’t seem very knowledgeable about ADHD. I needed to be convinced more, so I went to a psychologist who specializes in ADHD and took home surveys for me and my family to fill out independently. Then in a second appointment, we went over the surveys and he convinced me that I had ADHD and that I had a lot to gain by understanding it better. But a psychologist can’t prescribe drugs, so then I started working with a psychiatrist (could have gone to an MD as well). The psychiatrist didn’t need to see the diagnosis from the psychologist, they did their own 1/2 hour of questions, and made sure to rule out other things like bipolar.